He could be the first sitting MP put behind bars for 20 years when he is sentenced next month.

The disgraced Labour politician admitted three counts of “siphoning” £14,500 of taxpayers’ money. The case is almost certain to spark a by-election in Barnsley Central, which he has held since 1987 and retained last year with an 11,000 majority.

By parliamentary law, MPs jailed for more than a year must quit.

David Chaytor last week got 18 months for falsely claiming £22,000 in expenses but he was no longer a Labour MP when convicted.

Last night Illsley, 56, who sits as an independent after Labour sus-pended him, was under pressure to stand down whatever the sentence.

Party leader Ed Miliband said: “I don’t think he can be a credible voice for his constituents. He should do the right thing and resign.”

Illsley had earlier denied the false accounting charges on claims for his second home in London but yesterday changed his plea at Southwark crown court. Mr Justice Saunders adjourned the trial for a month for pre-sentence reports. Labour MP John Mann said it was “another bad day for Parliament” but hoped the case shows the pub-lic MPs are “not above the law”.

Simon Clements, head of the Crown Prosecution Service’s special crime division, said: “Illsley siphon-ed public money into his pockets and betrayed those who rightly expected the highest integrity from him as an MP.” TaxPayers’ Alliance director Matthew Sinclair said: “It is not fair ordinary families must fund luxury lifestyles and out-of-control costs for many MPs.

“People will be glad to see justice finally done but outraged any MP could dishonestly claim huge sums of their cash in the first place.”

The last sitting MP jailed was Labour’s Terry Field, given 60 days in 1991 for not paying the poll tax.